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In biology, a pair bond is the strong affinity that develops in some species between the male and female in a breeding pair. Pair-bonding, from 1940, is a term frequently used in sociobiology and evolutionary psychology circles and is typically meant to imply either a life-long monogamous relationship or a stage of mating interaction in socially monogamous species. It is sometimes used in reference to human relationships.

Pair bonding is also sometimes seen between individuals of the same sex, as demonstrated by behavior similar to that of male-female pair-bonded individuals. [1]

When discussing the social life of the bank swallow, Lipton and Barash state:

For about four days immediately prior to egg-laying, when copulations lead to fertilizations, the male bank swallow is very busy, attentively guarding his female. Before this time, as well as after—that is, when her eggs are not ripe, and again after his genes are safely tucked away inside the shells—he goes seeking extra-pair copulations with the mates of other males…who, of course, are busy with defensive mate-guarding of their own. It is unlikely that these chases are “sexual displays”, intending to enhance the pair-bond, as earlier literature in animal behavior has suggested. This is because:

males always chase females

males typically fight with other as an immediate result of such chases

when their own female is no longer fertile, mate males typically join in chases of other females

Thus, such males could not be solidifying an additional pair-bond, if only because no such “double-bonded” males have ever been found.

— Lipton and Barash, The Myth of Monogamy – Fidelity and Infidelity in Animals and People (2001)